A novel gene, algK, from the alginate biosynthesis cluster of Pseudomonas aeruginosa

Heterogenous Susceptibility to R-Pyocins in Populations of Pseudomonas aeruginosa Sourced from Cystic Fibrosis Lungs

Bacteriocins are proteinaceous antimicrobials produced by bacteria that are active against other strains of the same species. R-type pyocins are phage tail-like bacteriocins produced by Pseudomonas aeruginosa Due to their antipseudomonal activity, R-pyocins have potential as therapeutics in infection. P. aeruginosa is a Gram-negative opportunistic pathogen and is particularly problematic for individuals with cystic fibrosis (CF). P. aeruginosa organisms from CF lung infections develop increasing resistance to antibiotics, making new treatment approaches essential. P. aeruginosa populations become phenotypically and genotypically diverse during infection; however, little is known of the efficacy of R-pyocins against heterogeneous populations. R-pyocins vary by subtype (R1 to R5), distinguished by binding to different residues on the lipopolysaccharide (LPS). Each type varies in killing spectrum, and each strain produces only one R-type. To evaluate the prevalence of different R-types, we screened P. aeruginosa strains from the International Pseudomonas Consortium Database (IPCD) and from our biobank of CF strains. We found that (i) R1-types were the most prevalent R-type among strains from respiratory sources, (ii) a large number of strains lack R-pyocin genes, and (iii) isolates collected from the same patient have the same R-type. We then assessed the impact of intrastrain diversity on R-pyocin susceptibility and found a heterogenous response to R-pyocins within populations, likely due to differences in the LPS core. Our work reveals that heterogeneous populations of microbes exhibit variable susceptibility to R-pyocins and highlights that there is likely heterogeneity in response to other types of LPS-binding antimicrobials, including phage.IMPORTANCE R-pyocins have potential as alternative therapeutics against Pseudomonas aeruginosa in chronic infection; however, little is known about the efficacy of R-pyocins in heterogeneous bacterial populations. P. aeruginosa is known to become resistant to multiple antibiotics and to evolve phenotypic and genotypic diversity over time; thus, it is particularly difficult to eradicate in chronic cystic fibrosis (CF) lung infections. In this study, we found that P. aeruginosa populations from CF lungs maintain the same R-pyocin genotype but exhibit heterogeneity in susceptibility to R-pyocins from other strains. Our findings suggest there is heterogeneity in response to other types of LPS-binding antimicrobials, such as phage, highlighting the necessity of further studying the potential of LPS-binding antimicrobial particles as alternative therapies in chronic infections.

Synthesis and Antifungal activity of Phenacyl Azoles

A new N-(4-methoxyphenacyl)imidazole and three new substituted N-(phenacyl)triazoles were prepared by reaction of the heterocycle with a phenacyl halide. The former ketone and one example of the latter were reduced to the corresponding alcohols. All six compounds were screened in vitro for antifungal activity against two pathogenic fungal strains, Candida albicans (fluconazole-resistant) and Aspergillus fumigatus. The results revealed that most of the compounds showed activity against both strains at 100 μg mL-1 and 80 μg mL-1, some comparable with control compound fluconazole. The alcohols were less active than the corresponding ketones.

Biological 'glue' and 'Velcro': molecular tools for adhesion and biofilm formation in the hairy and gluey bug Pseudomonas aeruginosa

Pseudomonas aeruginosa contains an extraordinarily large number of loci encoding systems facilitating a communal lifestyle and binding to supports of various natures. These P. aeruginosa systems are reviewed here and may be categorized as classical or non-classical systems. They highlight the panoply of strategies that this hairy and gluey bacterium has developed for dealing with the diverse environments with which it is faced during various types of infection, involving complex regulatory networks that have not yet been fully elucidated but several aspects of which are discussed here.

AlgK is a TPR-containing protein and the periplasmic component of a novel exopolysaccharide secretin

The opportunistic pathogen Pseudomonas aeruginosa causes chronic biofilm infections in cystic fibrosis patients. During colonization of the lung, P. aeruginosa converts to a mucoid phenotype characterized by overproduction of the exopolysaccharide alginate. Here we show that AlgK, a protein essential for production of high molecular weight alginate, is an outer membrane lipoprotein that contributes to the correct localization of the porin AlgE. Our 2.5 A structure shows AlgK is composed of 9.5 tetratricopeptide-like repeats, and three putative sites of protein-protein interaction have been identified. Bioinformatics analysis suggests that BcsA, PgaA, and PelB, involved in the production and export of cellulose, poly-beta-1,6-N-Acetyl-D-glucosamine, and Pel exopolysaccharide, respectively, share the same topology as AlgK/E. Together, our data suggest that AlgK plays a role in the assembly of the alginate biosynthetic complex and represents the periplasmic component of a new type of outer membrane secretin that differs from canonical bacterial capsular polysaccharide secretion systems.

Molecular and bioengineering strategies to improve alginate and polydydroxyalkanoate production by Azotobacter vinelandii

Several aspects of alginate and PHB synthesis in Azotobacter vinelandii at a molecular level have been elucidated in articles published during the last ten years. It is now clear that alginate and PHB synthesis are under a very complex genetic control. Genetic modification of A. vinelandii has produced a number of very interesting mutants which have particular traits for alginate production. One of these mutants has been shown to produce the alginate with the highest mean molecular mass so far reported. Recent work has also shed light on the factors determining molecular mass distribution; the most important of these being identified as; dissolved oxygen tension and specific growth rate. The use of specific mutants has been very useful for the correct analysis and interpretation of the factors affecting polymerization. Recent scale-up/down work on alginate production has shown that oxygen limitation is crucial for producing alginate of high molecular mass, a condition which is optimized in shake flasks and which can now be reproduced in stirred fermenters. It is clear that the phenotypes of mutants grown on plates are not necessarily reproducible when the strains are tested in lab or bench scale fermenters. In the case of PHB, A. vinelandii has shown itself able to produce relatively large amounts of this polymer of high molecular weight on cheap substrates, even allowing for simple extraction processes. The development of fermentation strategies has also shown promising results in terms of improving productivity. The understanding of the regulatory mechanisms involved in the control of PHB synthesis, and of its metabolic relationships, has increased considerably, making way for new potential strategies for the further improvement of PHB production. Overall, the use of a multidisciplinary approach, integrating molecular and bioengineering aspects is a necessity for optimizing alginate and PHB production in A. vinelandii.

Bacterial alginates: from biosynthesis to applications

Alginate is a polysaccharide belonging to the family of linear (unbranched), non-repeating copolymers, consisting of variable amounts of beta-D-mannuronic acid and its C5-epimer alpha- L-guluronic acid linked via beta-1,4-glycosidic bonds. Like DNA, alginate is a negatively charged polymer, imparting material properties ranging from viscous solutions to gel-like structures in the presence of divalent cations. Bacterial alginates are synthesized by only two bacterial genera, Pseudomonas and Azotobacter, and have been extensively studied over the last 40 years. While primarily synthesized in form of polymannuronic acid, alginate undergoes chemical modifications comprising acetylation and epimerization, which occurs during periplasmic transfer and before final export through the outer membrane. Alginate with its unique material properties and characteristics has been increasingly considered as biomaterial for medical applications. The genetic modification of alginate producing microorganisms could enable biotechnological production of new alginates with unique, tailor-made properties, suitable for medical and industrial applications.

Understanding the control of Pseudomonas aeruginosa alginate synthesis and the prospects for management of chronic infections in cystic fibrosis

Decades of research have been dedicated to the study of the opportunistic pathogen Pseudomonas aeruginosa, a Gram-negative, environmental bacterium that secretes the exopolysaccharide alginate during chronic lung infection of cystic fibrosis (CF) patients. Although P. aeruginosa utilizes a variety of factors to establish a successful infection in the lungs of CF patients, alginate has stood out as one of the best-studied prognostic indicators of chronic lung infection. While the genetics, biosynthesis and regulation of alginate are well understood, questions still remain concerning its role in biofilm development and its potential as a therapeutic target. The purpose of this review is to provide a brief summary of alginate biosynthesis and regulation, and to highlight recent discoveries in the areas of alginate production, biofilm formation and vaccine design. This information is placed in context with a proposed P. aeruginosa infectious pathway, highlighting avenues for the use of existing therapies as well as the potential for novel agents to reduce or eliminate chronic infections in CF patients.

α-Substituted hydroxamic acids as novel bacterial deformylase inhibitor-based antibacterial agents

We report the synthesis and biological activity of analogues of VRC3375 (N-hydroxy-3-R-butyl-3-[(2-S-(tert-butoxycarbonyl)-pyrrolidin-1-ylcarbonyl]propionamide), an orally active peptide deformylase inhibitor. This study explores the structure-activity relationship of various chelator groups, alpha substituents, P(2)' and P(3)' substituents in order to achieve optimal antibacterial activity with minimal toxicity liability.

The Pseudomonas fluorescens AlgG protein, but not its mannuronan C-5-epimerase activity, is needed for alginate polymer formation

Bacterial alginates are produced as 1-4-linked beta-D-mannuronan, followed by epimerization of some of the mannuronic acid residues to alpha-L-guluronic acid. Here we report the isolation of four different epimerization-defective point mutants of the periplasmic Pseudomonas fluorescens mannuronan C-5-epimerase AlgG. All mutations affected amino acids conserved among AlgG-epimerases and were clustered in a part of the enzyme also sharing some sequence similarity to a group of secreted epimerases previously reported in Azotobacter vinelandii. An algG-deletion mutant was constructed and found to produce predominantly a dimer containing a 4-deoxy-L-erythro-hex-4-enepyranosyluronate residue at the nonreducing end and a mannuronic acid residue at the reducing end. The production of this dimer is the result of the activity of an alginate lyase, AlgL, whose in vivo activity is much more limited in the presence of AlgG. A strain expressing both an epimerase-defective (point mutation) and a wild-type epimerase was constructed and shown to produce two types of alginate molecules: one class being pure mannuronan and the other having the wild-type content of guluronic acid residues. This formation of two distinct classes of polymers in a genetically pure cell line can be explained by assuming that AlgG is part of a periplasmic protein complex.

Hexuronyl C5-epimerases in alginate and glycosaminoglycan biosynthesis

The sugar residues in most polysaccharides are incorporated as their corresponding monomers during polymerization. Here we summarize the three known exceptions to this rule, involving the biosynthesis of alginate, and the glycosaminoglycans, heparin/heparan sulfate and dermatan sulfate. Alginate is synthesized by brown seaweeds and certain bacteria, while glycosaminoglycans are produced by most animal species. In all cases one of the incorporated sugar monomers are being C5-epimerized at the polymer level, from D-mannuronic acid to L-guluronic acid in alginate, and from D-glucuronic acid to L-iduronic acid in glycosaminoglycans. Alginate epimerization modulates the mechanical properties of seaweed tissues, whereas in bacteria it seems to serve a wide range of purposes. The conformational flexibility of iduronic acid units in glycosaminoglycans promotes apposition to, and thus functional interactions with a variety of proteins at cell surfaces and in the extracellular matrix. In the bacterium Azotobacter vinelandii the alginates are being epimerized at the cell surface or in the extracellular environment by a family of evolutionary strongly related modular type and Ca(2+)-dependent epimerases (AlgE1-7). Each of these enzymes introduces a specific distribution pattern of guluronic acid residues along the polymer chains, explaining the wide structural variability observed in alginates isolated from nature. Glycosaminoglycans are synthesized in the Golgi system, through a series of reactions that include the C5-epimerization reaction along with extensive sulfation of the polymers. The single, Ca(2+)-independent, epimerase in heparin/heparan sulfate biosynthesis and the Ca(2+)-dependent dermatan sulfate epimerase(s) also generate variable epimerization patterns, depending on other polymer-modification reactions. The alginate and heparin epimerases appear unrelated at the amino acid sequence level, and have probably evolved through independent evolutionary pathways; however, hydrophobic cluster analysis indicates limited similarity. Seaweed alginates are widely used in industry, while heparin is well established in the clinic as an anticoagulant.

Involvement of the exopolysaccharide alginate in the virulence and epiphytic fitness of Pseudomonas syringae pv. syringae

Alginate, a co-polymer of O-acetylated beta-1,4-linked D-mannuronic acid and L-guluronic acid, has been reported to function in the virulence of Pseudomonas syringae, although genetic studies to test this hypothesis have not been undertaken previously. In the present study, we used a genetic approach to evaluate the role of alginate in the pathogenicity of P. syringae pv. syringae 3525, which causes bacterial brown spot on beans. Alginate biosynthesis in strain 3525 was disrupted by recombining Tn5 into algL, which encodes alginate lyase, resulting in 3525.L. Alginate production in 3525.L was restored by the introduction of pSK2 or pAD4033, which contain the alginate biosynthetic gene cluster from P. syringae pv. syringae FF5 or the algA gene from P. aeruginosa respectively. The role of alginate in the epiphytic fitness of strain 3525 was assessed by monitoring the populations of 3525 and 3525.L on tomato, which is not a host for this pathogen. The mutant 3525.L was significantly impaired in its ability to colonize tomato leaves compared with 3525, indicating that alginate functions in the survival of strain 3525 on leaf surfaces. The contribution of alginate to the virulence of strain 3525 was evaluated by comparing the population dynamics and symptom development of 3525 and 3525.L in bean leaves. Although 3525. L retained the ability to form lesions on bean leaves, symptoms were less severe, and the population was significantly reduced in comparison with 3525. These results indicate that alginate contributes to the virulence of P. syringae pv. syringae 3525, perhaps by facilitating colonization or dissemination of the bacterium in planta.

Regulation of alginate biosynthesis in Pseudomonas syringae pv. syringae

Both Pseudomonas aeruginosa and the phytopathogen P. syringae produce the exopolysaccharide alginate. However, the environmental signals that trigger alginate gene expression in P. syringae are different from those in P. aeruginosa with copper being a major signal in P. syringae. In P. aeruginosa, the alternate sigma factor encoded by algT (sigma22) and the response regulator AlgR1 are required for transcription of algD, a gene which encodes a key enzyme in the alginate biosynthetic pathway. In the present study, we cloned and characterized the gene encoding AlgR1 from P. syringae. The deduced amino acid sequence of AlgR1 from P. syringae showed 86% identity to its P. aeruginosa counterpart. Sequence analysis of the region flanking algR1 in P. syringae revealed the presence of argH, algZ, and hemC in an arrangement virtually identical to that reported in P. aeruginosa. An algR1 mutant, P. syringae FF5.32, was defective in alginate production but could be complemented when algR1 was expressed in trans. The algD promoter region in P. syringae (PsalgD) was also characterized and shown to diverge significantly from the algD promoter in P. aeruginosa. Unlike P. aeruginosa, algR1 was not required for the transcription of algD in P. syringae, and PsalgD lacked the consensus sequence recognized by AlgR1. However, both the algD and algR1 upstream regions in P. syringae contained the consensus sequence recognized by sigma22, suggesting that algT is required for transcription of both genes.

Bacterial alginate biosynthesis--recent progress and future prospects

The extracellular polysaccharide alginate has been widely associated with chronic Pseudomonas aeruginosa infections in the cystic fibrosis lung. However, it is clear that alginate biosynthesis is a more widespread phenomenon. Alginate plays a key role as a virulence factor of plant-pathogenic pseudomonads, in the formation of biofilms and with the encystment process of Azotobacter spp.

Deletion of algK in mucoid Pseudomonas aeruginosa blocks alginate polymer formation and results in uronic acid secretion

Chronic pulmonary infection with Pseudomonas aeruginosa is a common and serious problem in patients with cystic fibrosis (CF). The P. aeruginosa isolates from these patients typically have a mucoid colony morphology due to overproduction of the exopolysaccharide alginate, which contributes to the persistence of the organisms in the CF lung. Most of the alginate biosynthetic genes are clustered in the algD operon, located at 34 min on the chromosome. Alginate biosynthesis begins with the formation of an activated monomer, GDP-mannuronate, which is known to occur via the products of the algA, algC, and algD genes. Polymannuronate forms in the periplasm, but the gene products involved in mannuronate translocation across the inner membrane and its polymerization are not known. One locus of the operon which remained uncharacterized was a new gene called algK between alg44 and algE. We sequenced algK from the mucoid CF isolate FRD1 and expressed it in Escherichia coli, which revealed a polypeptide of the predicted size (52 kDa). The sequence of AlgK showed an apparent signal peptide characteristic of a lipoprotein. AlgK-PhoA fusion proteins were constructed and shown to be active, indicating that AlgK has a periplasmic subcellular localization. To test the phenotype of an AlgK-mutant, the algK coding sequence was replaced with a nonpolar gentamicin resistance cassette to avoid polar effects on genes downstream of algK that are essential for polymer formation. The algKdelta mutant was nonmucoid, demonstrating that AlgK was required for alginate production. Also, AlgK- mutants demonstrated a small-colony phenotype on L agar, suggesting that the loss of AlgK also caused a growth defect. The mutant phenotypes were complemented by a plasmid expressing algK in trans. When the algKdelta mutation was placed in an algJ::Tn501 background, where algA was not expressed due to polar transposon effects, the growth defect was not observed. AlgK- mutants appeared to accumulate a toxic extracellular product, and we hypothesized that this could be an unpolymerized alginate precursor. High levels of low-molecular-weight uronic acid were produced by the AlgK- mutant. When AlgK- culture supernatants were subjected to dialysis, high levels of uronic acids diffused out of the dialysis sac, and no uronic acids were detectable after extensive dialysis. In contrast, the mucoid wild-type strain produced only polymerized uronic acids (i.e., alginate), whereas the algKdelta algJ::Tn501 mutant produced no uronic acids. Thus, the alginate pathway in an AlgK- mutant was blocked after transport but at a step before polymerization, suggesting that AlgK plays an important role in the polymerization of mannuronate to alginate.

Isolation and characterization of an Azotobacter vinelandii algK mutant

Random Tn5 mutagenesis over Azotobacter vinelandii mucoid strain ATCC 9046 produced strain LA21, a non-mucoid, non-encysting mutant, carrying the Tn5 insertion within a gene homologous to algK from Pseudomonas aeruginosa encoding a periplasmic protein. algK, algJ and algG were shown to be transcribed as part of the palg8-alg44-algK-algJ-algG operon. A non-polar algK mutant was constructed and showed a non-mucoid phenotype, indicating that algK is essential for alginate production.